The present invention is related a water based ink for ball-point pen (bpp). This expression indicates a writing instrument including structural components, such as a body retaining an ink reservoir, as well as means for delivering the ink to a socket located at one end of the instrument, and a routable point retained in the socket, suitable to apply the ink delivered to the socket to a surface, providing visible markings on the same surface. Only the portion of ink required is fed to the ball from the ink's reservoir.
A water based ink has a low viscosity, and is generally fed making use of the principle of capillarity. In particular, a conventional ball-point pen for water based inks has an absorbent fibrous ink reservoir and its ink, having a viscosity comprised between 3 and 5 mPa.s, is fed to the pen body through an absorbent feeding rod.
Undesirable variations in thickness of writing take frequently place. Also, it is difficult to know the amount of ink left in the pen. The holding capacity is in any case small. Moreover, the continuity of the ink column, in the pen body may be broken by solvent evaporation from the ball-point. Thus many bpp for water-based inks include a cap member to seal the ball-point and the tip, in order to control evaporation of volatile components of the ink and leakage of air around the ball into the ink reservoir, which may cause formation of gas bubble which can cause breakage of contact between the column of ink and the ball, with interruption of writing.
Rheologic properties are critical in the ink performance, because they can exert a direct influence on ink properties and on writing quality. Thus, in recent years water based inks, which have a pseudoplastic behaviour due to the addition of suitable substances, were produced.
During writing pseudoplastic inks have a reduced viscosity, because a shear force is applied to the ink, due to the rotation of the ball at the tip point. On the contrary, the same inks have a high viscosity when they are not subjected to shear forces, so as they do not flow from the relative pen when not in use.
The ball-point pens using this pseudoplastic water based ink can store it directly into the ink reservoir, without absorbing sector, and that simplify its structure. Moreover, the use of transparent materials makes possible to check the remaining amount of ink in the ink reservoir.
However ordinary thickening agents with pseudoplastic behaviour employed in inks' composition could not give, up to now, completely satisfactory results.
For example, Hydroxy Ethyl-Cellulose (HEC) can give to the ink a viscosity that may reach even 100 mPa.s at a shear rate of 30 s−1, and maximum values of about 100 mPa.s at the shear rate produced by writing, which is of the order of 1000 s−1. This is not satisfactory since these values (which are index of a pseudo-newtonian behaviour) bring about poor ink's flow in fast writing, and, on the contrary, exceeding flow, with ink dripping, when the pen is not in use. Moreover, this fact causes a capillary absorption of ink on the paper during writing pauses.
When only pseudoplastic additives are used, such as polyacrilic acid salts, cross linking type acrylic acid polymers, salts of styrene-acrylic acid copolymer, salts of styrene-maleic acid copolymer, non ionic polymers such as polyvinylpirrolodone and polyethylene glycol, or only polysaccharides such as xantan gum, alginic acid, the ink containing these viscosity controlling agents has a viscosity falling to 100 mPa.s, when the shear rate is 3,84 s−1. That is not satisfying again because, when the pen does not work, the ink tends to drip, especially at temperatures over 30° C. or in particular barometric conditions (depressurisation).
Also when dextrins or maltodextrins are used as viscosity control agents, ink's viscosity may fall to about 100 mPa.s when the shear rate is 3,84 s−1. That is not satisfying because, also in this case, the ink tends to drip, when the pen does not work.